Formation in Amphibians July 9, 2009 The Amphibian Model Common - - PowerPoint PPT Presentation
Biology 4361 Early Development and Axis Formation in Amphibians July 9, 2009 The Amphibian Model Common vertebrate (e.g. Rana , Xenopus ) Manipulable, observable (i.e. large eggs and embryos) 1) How are the body axes established? - How do
1) How are the body axes established?
establish polarity (complexity from simplicity)? 2) How are the germ layers determined?
(i.e. blastomeres) down pathways toward different fates? 3) How is development organized in a regulative system? Common vertebrate (e.g. Rana, Xenopus) Manipulable, observable (i.e. large eggs and embryos)
(Photo by Harland lab/UC Berkeley)
Merobl blas asti tic (incom
plete ete cleav avage) age) Holobl blas asti tic (complete ete cleav avage) age) cephalopod molluscs fish, reptiles, birds most insects echinoderms, amphioxis annelids, molluscs, flatworms tunicates mammals, nematode amphibians Spec ecies es Spiral Bilateral Radial Rotational Displaced radial Bilateral Discoidal Superficial Cleavage age Yolk clas assificati ation
Isolecithal Telolecithal Centrolethical Mesolecithal
Cell cycles regulated by mitosis-promoting factor (MPF)
… until mid-blastula blastula transi nsition tion
Formation of the dorsal lip
Involuting Marginal Zone
motive force (D)
Movements
convergent extension
NIMZ
(Photo by Harland lab/UC Berkeley)
2-cell stage
If one blastomere received no gray crescent material, resulted in
mesenchyme, gut cells
(e.g. notochord, somites)
The gray crescent area is critical for proper development Gray crescent = future dors rsal lip of th the blast stopo
Doral l lip = “Organiz izer er”
(Spemann, Mangold experiment)
(Photo by Harland lab/UC Berkeley)
Transplantation experiments established the organizing properties
l blast stopo
e lip, which … 2) Establishes the dorsal-ventral axis 3) Specifies multiple tissues, including… 1) Self-differentiates (all other tissues conditionally specified)
4) Dorsalizes surrounding mesoderm into paraxial mesoderm 5) Induces the neural tube 6) Initiates the movements of gastrulation How is the dorsal al lip specifi ified ed?
β-catenin is initially distributed throughout the embryo,
Organizer Nieuwkoop center β-catenin
Dorsalization of β-catenin: a) protect β-catenin in dorsal area, b) degrade β-catenin everywhere else. Mechanism - cortical rotation … (in sea urchins, specifies micromeres, endomesoderm) nuclear transcription factor (in Wnt pathway) β-cate ateni nin (in Xenopus, specifies dorsal structures; e.g. organizer)
β-ca caten enin in is initially distributed throughout the oocyte β-catenin induces cells to dorsal fates GSK3 marks β-catenin for degradation Glyc ycoge
n synt nthase hase kinase ase 3 (GSK3) is also distributed throughout oocyte Dish shevel evelled led (Dsh Dsh) blocks GSK3 activity Dsh localizes in the cytoplasmic cortex at the vegetal pole
β-ca caten enin in, GSK3 distributed throughout GBP – GSK3 binding protein Dsh Dsh – Dishevelled Kines esin in - motor protein
Dish shevel evelled led - blocks GSK3-mediated β-catenin degradation
but Disheveled and GBP block GSK3;
the marginal area opposite the point
GSK3 mediates β-catenin destruction ……
At the blastula stage, β-catenin is located exclusively in the future dorsal region
β-catenin acts with Tcf3 (transcription factor); stimulates expression of dorsalizing genes: Siamois mois – TF; activates Xlim, goosecoid (dorsal determinants) Goosecoid ecoid protein – TF responsible for
Goosecoid also plays a part in specifying dorsal al me mesoderm erm; however, additional vegetal factors are needed: e.g. Vegeta etal l TGF-β signals als
(mid-blastula)
VegT, Vg1, Xnr(s) (s) – Xenopu pus nodal dal relat ated ed genes; es; TGF-β family
(late blastula) (early gastrulation) , dorsal and lateral mesoderm
muscle, kidney & intermediate
Later gastrula cells are determined
development Early gastrulation stage cells are uncommitted
development
D-V axis – set up at fertilization A-P axis – established by gastrulation movements across the dorsal lip of the blastopore L-R axis – Nodal expression on Left, not Right
Left–Right axis established by Xenopus nodal-related (Xnr1) Xnr1 expression is limited to the left side in a process involving cortical rotation and Vg1 gut coiling – counter-clockwise heart loops to the left Block Xnr1 expression = random gut coiling, heart looping Nodal al expression:
e! wild-type Xnr1-/-